Field Of The Invention
The present invention relates to improvements in the manufacture of cellulosic pulp sheets.
Description Of The Background
Cellulosic pulp is generally manufactured in pulp mills or integrated mills that serve as both pulp and paper mills. Normally wood and/or other fibrous cellulosic feed-stock is broken up to form a cellulosic pulp, which is usually subjected to various washing and filtering stages. Additionally the pulp may also be bleached. In an integrated mill it is unnecessary to dry the pulp at any stage and instead may be diluted directly to form a thin stock for the papermaking process.
Pulp mills that are not integrated into paper mills also manufacture the pulp from wood or fibrous cellulosic material which is then converted to a dry product generally known as “dry market pulp”. This dry pulp may then be used as a feedstock at a paper mill to make the aqueous cellulosic suspension used in a papermaking process.
The pulping stages in a pulp mill can generally be similar to the pulping stages in an integrated mill except that at the end of the washing stages it is necessary to drain the pulp and then thermally dry it. This drainage may often be conducted on a machine known as a “lap pulp machine”.
Japanese patent publication 59-087097 describes the vacuum dehydration of sludge containing crushed matter of pulp containing cellulosic material using generally a cationic macromolecular coagulant, for instance cationically modified polyacrylamide, chitosan, and polyvinyl imidazoline.
EP 335576 sets out to improve the drain- age in a process for making dry market pulp. It is indicated that previously the addition of sophisticated dewatering and retention systems in pulp mills had been found unsuccessful due to reductions in drainage and the increase in the amount of thermal drying would be required produce the dried pulp sheets. The inventors of that disclosure describe a pulp making process in which a water-soluble cationic polymer is added to the suspension of cellulosic material before one or more shear stages and then after that shear stage the addition of an inorganic material such as bentonite. The document exemplifies the use of a copolymer of 70% by weight acrylamide and 30% by weight (13.6 mole %) dimethyl amino ethyl acrylate quaternised with methyl chloride of intrinsic viscosity 10 dl/g in conjunction with bentonite. Also exemplified are polymers with the same monomer units and cationicity but with intrinsic viscosities of 8 to 10 dl/g and 6 to 8 dl/g respectively and the test work indicates improved dewatering time when these two polymers are used in conjunction with bentonite by comparison to the use of the polymers alone.
More recently WO 02/088468 describes a method for the production of shock resistant fibrous moulded bodies. The process involves the addition of a modified starch to an aqueous mass of fibrous material before it is placed into a mould. The modified starch is prepared by digesting starch in the presence of at least one cationic polymer.
WO 2008/036031 relates to a method for preparing pulp sheets involving treating an aqueous suspension of bleached pulp derived from an alkaline pulping process involving dewatering and drying the suspension, in which the pH of the suspension is between 6.5 and 12. The use of cationic starch or cationic polyacrylamide is described for the dewatering.
However, there is a desire to further improve the drainage rate and dryness of the resulting dewatered pulp sheets.